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changed. When it strikes the quartz the ray is broken up i
the two components oy and Ox. These traverse the quartz
with different velocities, and since one ray is retarded half **
wave-length in respect of the other, the vibration of one com-
ponent will be represented by oy, but the other must be i*e-
presented by Ox' instead of Ox. These two combine ojl
emerging to a plane polarised ray vibrating in the direction
OB' so that the angle AOB' is equal to the angle AOB.

If now (the tube containing water or other non-rotating
liquid) the nicol N be so placed that it is parallel to nicol 1%
then the light, in the half of the field to the right, will pass
through unchanged, but only a portion of the light which. l^^iS

FIG. 73
passed through the quartz diaphragm with its plane of vibration
in the direction OB', will pass through N ancl consequently trie re
will be different intensities of illumination in the two halves*
of the field, Fig. 73 b (if the angle a is 45 then the angle ISO I*
will be 90, and the light in the left half of the field will be com-
pletely obscured). Similarly if the plane of the nicol N be rnncle
parallel to OB'there will be a greater intensity of illumination
in the left half of the field, Fig. 73 c. Between the two positions
of the nicol N there must necessarily be one which gfives
uniform illumination of the whole field, and this is the 2:0ro
point of the instrument, Fig 73 d.
If the tube T, containing the active substance, be interposed
between the two nicols, then both rays OB and OB' will "be
rotated through equal angles, and to re-establish uniform